Failure modes — white-layer spall, network cracking, distortion, embrittlement

Visual: parts out of tolerance after nitriding, with the dimensional error often largest in long thin features and on asymmetric geometry. A 12" punch may grow 0.001-0.003" overall length and bow 0.0005-0.002" off straight.

Three drivers. Residual machining stress relieves at nitriding temperature; rough-machined parts that skipped stress relief move the most. Thermal cycling through the substrate's temper temperature contributes if the cycle drifts within 20-30°C of the temper. Non-uniform case growth on asymmetric geometry creates asymmetric compressive surface stresses that bow the part.

The fix sequence is straightforward. Stress-relieve before nitriding, at 30-50°C above the planned nitride temperature, after rough machining and any EDM. For tight-tolerance work, choose plasma over gas; the lower processing window (480-520°C versus 540-570°C for gas) cuts the thermal swing and is the reason precision gear shops default to plasma. If the tolerance budget is tight enough that any distortion is unacceptable, finish-machine after nitriding, but only on a recipe that produces compound-layer-free diffusion zone (low Kn or plasma at near-α conditions), because finish-machining through a compound layer wrecks the surface that gave the part its life in the first place.

A realistic distortion budget for a well-controlled nitride job on a long thin component runs 0.0005-0.002" per inch of length. Tighter than that requires post-nitride finish grinding within case-depth-conscious stock allowances (Lesson 7 covers grinding limits).